With the rapid spread of portable electronic equipment, the specifications required of the batteries used in such equipment have become more stringent with every year, and there is particular requirement for batteries that are compact and thin, have high capacity and superior cycling characteristics, and give stable performance. In the field of secondary batteries, attention is focusing on nonaqueous electrolyte secondary batteries, which have high energy density compared with other batteries. These nonaqueous electrolyte secondary batteries are winning an increasingly large share of the secondary battery market.
In the equipment that uses such nonaqueous electrolyte secondary batteries, the space for housing the battery is often square (flattened box shape), so that the nonaqueous electrolyte secondary battery used is often formed so as to be square and to have its generating elements housed in a square case. An example of such a square nonaqueous electrolyte secondary battery will now be described using the accompanying drawing. FIG. 1 is a perspective view of a related art square nonaqueous electrolyte secondary battery, cut in the longitudinal direction. This nonaqueous electrolyte secondary battery 10 has a flattened electrode roll 14 in which a positive electrode plate 11 and a negative electrode plate 12 are rolled up with separators 13 interposed therebetween, and which is housed inside a square battery case 15. The battery case 15 is sealed by a sealing plate 16. The electrode roll 14 is rolled so that the positive electrode plate 11 is located on the outermost circumference and is exposed. The exposed, outer-circumference positive electrode plate 11 contacts directly, and thus is electrically coupled, with the inner surface of the battery case 15, which serves also as a positive electrode terminal. The negative electrode plate 12 is electrically coupled via a collector 19 to a negative electrode terminal 18 that is installed in the center of the sealing plate 16 with an insulator 17 interposed.
Since the battery case 15 is electrically coupled to the positive electrode plate 11, an insulating spacer 20 is inserted between the top end of the electrode roll 14 and the sealing plate 16, thereby electrically insulating the negative electrode plate 12 from the battery case 15, in order to prevent short circuiting between the negative electrode plate 12 and the battery case 15. To fabricate this square nonaqueous electrolyte secondary battery, the electrode roll 14 is inserted inside the battery case 15, then the sealing plate 16 is laser-welded over the open portion of the battery case 15, a nonaqueous electrolyte is poured in through an electrolyte pouring hole 21, and the electrolyte pouring hole 21 is sealed. Such a square nonaqueous electrolyte secondary battery wastes little space when used, and has the excellent advantages of high battery performance and high battery reliability.
The negative electrode active materials that are widely used in such square nonaqueous electrolyte secondary batteries are graphite, amorphous carbon and other carbonaceous materials, which, while having discharge potential rivaling that of lithium metals and lithium compounds, are not prone to dendrite growth, and therefore are high in safety and have superior initial efficiency and good potential flatness, as well as the excellent quality of high density.
As regards the positive electrode active materials, it is known that using a lithium composite oxide such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4 or LiFeO2 in combination with a negative electrode constituted of a carbon material will give a high energy density 4 V class nonaqueous electrolyte secondary battery. Of these, LiCoO2 is in frequent use because it provides various battery characteristics that are superior to those provided by the others.
For the nonaqueous electrolyte's nonaqueous solvent, use is made of carbonate, lactone, ether or ester, either singly or in a mixture of two or more. Of these, carbonate, which has high permittivity and high electrolyte ion conductivity, is in particularly frequent use.
In the event of overcharging or short-circuiting, the nonaqueous electrolyte of such nonaqueous electrolyte secondary batteries will reach high temperatures, which may result in generation of gas, or in swelling, ignition or explosion of the battery, etc. Accordingly, various additives are used simultaneously in combination with the nonaqueous electrolyte in order to assure safety. In JP-2004-214139-A for example, an electrolyte is disclosed in which a cyclic carbonate ester—an unsaturated hydrocarbon—is used as the nonaqueous solvent, and the additive that is added to assure safety in the event of overcharging contains at least one of the set composed of cyclohexylbenzene (CHB) and its derivatives, and at least one of the set composed of vinylene carbonate (VC), vinyl ethylene carbonate, and their derivatives.
Also, JP-2004-349131-A discloses a nonaqueous electrolyte to which an aromatic compound given by chemical formula (II) below is added as an additive for preventing overcharging.
[Chemical Formula 4]
(where: R1 and R2 represent alkyl groups that are each separated and may have substitutional groups, or else R1 and R2 are bonded to each other and form hydrocarbon rings that may have substitutional groups. Ring A may have substitutional groups, and at least one of the carbon atoms adjacent to the carbon atom with which R1R2CH— bonds must have a substitutional group.) Also, JP-2003-272700-A discloses that when a pentafluorophenol compound given by chemical formula (III) below is added to the nonaqueous electrolyte, a nonaqueous electrolyte secondary battery is obtained that has superior cycling characteristics and charging/storing characteristics and whose 50-cycle discharge capacity maintenance rate is 88% or higher.[Chemical Formula 5]
(where: R represents a substitutional group selected from a set composed of an alkyl carbonyl group with carbon number 2 to 12, an alkoxycarbonyl group with carbon number 2 to 12, an aryloxy carbonyl group with carbon number 7 to 18, and an alkane sulfonyl group with carbon number 1 to 12. At least one of the hydrogen atoms possessed by such a substitutional group may be substituted with a halogen atom or an aryl group with carbon number 6 to 18.)
Further, JP-A-2004-519829 discloses that when pentafluoroanisol or other fluorobenzene composition such as given by chemical formula (IV) below is added to the nonaqueous electrolyte, a nonaqueous electrolyte secondary battery is obtained that has a large reversible fraction and high cycling life.
[Chemical Formula 6]
(where: R1 and R2 are separated and are hydrogen, halogen or another electron attracting withdrawing substituent or electron donating substituent. If R1 is a non-halogen electron withdrawing substituent, R2 must be an electron donating substituent.)
With the higher performance levels of portable equipment in recent years, further enhancement of secondary battery capacity has come to be expected of such nonaqueous electrolyte secondary batteries as well. Commonly known means of meeting such demand are to use higher-density electrode materials, to use thin films for the current collectors and separators, etc., and to use high charging voltage for the battery voltage. Of these means, the use of higher-density electrode materials and the use of thin films for the collectors and separators pose major problems of lowered productivity. By contrast, the use of high charging voltage for the battery voltage has minimal impact on productivity and enables high capacity to be achieved, and therefore will be an essential technique for the future development of high capacity batteries.
For example, in a nonaqueous electrolyte secondary battery that uses as its positive electrode active material a lithium-containing transition metal oxide such as the aforementioned lithium cobalt oxide LiCoO2, and as its negative electrode active material a carbon material, the charging voltage is usually 4.1 to 4.2 V (the potential of the positive electrode active material being 4.2 to 4.3 V relative to lithium) when combined with the negative electrode active material of graphite or other carbon material. With such charging condition, only 50% to 60% of the theoretical capacity of the positive electrode can be utilized. Thus, if the charging voltage can be rendered higher, it will be possible to utilize 70% or more of the theoretical capacity of the positive electrode, thereby rendering the battery high-capacity and high-energy density.
In order to obtain a positive electrode active material that could stably achieve a high charging voltage, the present applicant engaged repeatedly in various investigations, and as a result developed a new nonaqueous electrolyte secondary battery that uses as its positive electrode active material a mixture of lithium cobalt oxide with a dissimilar element added, plus layered lithium manganese-nickel oxide, and that has been disclosed already in JP-2005-317499-A. With the positive electrode active material of the nonaqueous electrolyte secondary battery disclosed in JP-2005-317499-A, structural stability at high voltage (up to 4.5 V) is enhanced by the addition of at least Zr and Mg as dissimilar elements to the lithium cobalt oxide, and furthermore safety is assured through the presence of layered lithium manganese-nickel oxide, which has high thermal stability under high voltage, in the mixture. By combining a positive electrode that uses such positive electrode active material with a negative electrode having negative electrode active material constituted of carbon material, there is obtained a nonaqueous electrolyte secondary battery that is chargeable at high charging voltage of 4.3 to 4.5 V (charging termination potential being 4.4 to 4.6 V relative to lithium).
However, when the charging voltage is made high, as in the nonaqueous electrolyte secondary battery disclosed in JP-2005-317499-A, it is not possible to use overcharging protection additives such as biphenyl (BP) or CHB which have been widely used with related art 4.2 V charging voltage lithium batteries, because such additives would decompose and give rise to side reactions during normal use. Accordingly, in related art high charging voltage nonaqueous electrolyte secondary batteries, a heat-sensing protective element has been installed to assure safety in the event of overcharging.
In order to assure safety in the event of overcharging of the high charging voltage nonaqueous electrolyte secondary battery, it will be necessary to add a compound that has decomposition potential higher than that of BP or CHB, and that moreover has properties that will deactivate the battery before abnormality develops. Besides effects due to overcharging, it will also be important that no adverse effects are exerted on the battery characteristics during normal use.